Object-oriented technology (OOT), which generally includes object-oriented analysis (OOA), object-oriented design (OOD), and object-oriented programming (OOP), is earning its place as one of the most important new technologies in software development. OOT has already begun to prove its ability to create significant increases in programmer productivity and in program maintainability. By engendering an environment in which data and the procedures that operate on the data are combined into packages called objects, and by adopting a rule that demands that objects communicate with one another only through well-defined messaging paths, OOT removes much of the complexity of traditional, procedure-oriented programming.
The following paragraphs present a brief overview of some of the more important aspects of OOT. More detailed discussions of OOT are available in many publicly available documents, including Object Oriented Design With Applications by Grady Booch (Benjamin/Cummings Publishing Company, 1991) and Object-Oriented Requirements Analysis and Logical Design by Donald G. Firesmith (John Wiley & Sons, Inc., 1993). The basic component of OOT is the object. An object includes, and is characterized by, a set of data (also called attributes) and a set of operations (called methods) that can operate on the data. Generally, an object's data may change only through the operation of the object's methods.
A method in an object is invoked by passing a message to the object (this process is called message passing). The message specifies a method name and an argument list. When the object receives the message, code associated with the named method is executed with the formal parameters of the method bound to the corresponding values in the argument list. Methods and message passing in OOT are analogous to procedures and procedure calls in procedure-oriented software environments. However, while procedures operate to modify and return passed parameters, methods operate to modify the internal state of the associated objects (by modifying the data contained therein). The combination of data and methods in objects is called encapsulation. Perhaps the greatest single benefit of encapsulation is the fact that the state of any object can only be changed by well-defined methods associated with that object. When the behavior of an object is confined to such well-defined locations and interfaces, changes (that is, code modifications) in the object will have minimal impact on the other objects and elements in the system. A second "fringe benefit" of good encapsulation in object-oriented design and programming is that the resulting code is more modular and maintainable than code written using more traditional techniques.
The fact that objects are encapsulated produces another important fringe benefit that is sometimes referred to as data abstraction. Abstraction is the process by which complex ideas and structures are made more understandable by the removal of detail and the generalization of their behavior. From a software perspective, abstraction is in many ways the antithesis of hard-coding. Consider a software windowing example: if every detail of every window that appears on a user's screen in a graphical user interface (GUI)-based program had to have all of its state and behavior hard-coded into a program, then both the program and the windows it contains would lose almost all of their flexibility. By abstracting the concept of a window into a window object, object-oriented systems permit the programmer to think only about the specific aspects that make a particular window unique. Behavior shared by all windows, such as the ability to be dragged and moved, can be shared by all window objects.
This leads to another basic component of OOT, which is the class. A class includes a set of data attributes plus a set of allowable operations (that is, methods) on the data attributes. Each object is an instance of some class. As a natural outgrowth of encapsulation and abstraction, OOT supports inheritance. A class (called a subclass) may be derived from another class (called a base class, a parent class, etc.) wherein the subclass inherits the data attributes and methods of the base class. The subclass may specialize the base class by adding code which overrides the data and/or methods of the base class, or which adds new data attributes and methods. Thus, inheritance represents a mechanism by which abstractions are made increasingly concrete as subclasses are created for greater levels of specialization. Inheritance is a primary contributor to the increased programmer efficiency provided by OOP. Inheritance makes it possible for developers to minimize the amount of new code they have to write to create applications. By providing a significant portion of the functionality needed for a particular task, classes in the inheritance hierarchy give the programmer a head start to program design and creation. One potential drawback to an objectoriented environment lies in the proliferation of objects that must exhibit behavior which is similar and which one would like to use as a single message name to describe. Consider, for example, an object-oriented graphical environment: if a Draw message is sent to a Rectangle object, the Rectangle object responds by drawing a shape with four sides. A Triangle object, on the other hand, responds by drawing a shape with three sides. Ideally, the object that sends the Draw message remains unaware of either the type of object to which the message is addressed or of how that object that receives the message will draw itself in response. If this ideal can be achieved, then it will be relatively simple to add a new kind of shape later (for example, a hexagon) and leave the code sending the Draw message completely unchanged.
In conventional procedure-oriented languages, such a linguistic approach would wreak havoc. In OOT environments, the concept of polymorphism enables this to be done with impunity. As one consequence, methods can be written that generically tell other objects to do something without requiring the sending object to have any knowledge at all about the way the receiving object will understand the message. Software programs, be they object-oriented, procedure-oriented, rule based, etc., almost always interact with the operating system to access the services provided by the operating system. For example, a software program may interact with the operating system in order to access data in memory, to receive information relating to processor faults, to communicate with other processes, or to schedule the execution of a process.
Most conventional operating systems are procedure-oriented and include native procedural interfaces. Consequently, the services provided by these operating systems can only be accessed by using the procedures defined by their respective procedural interfaces. If a program needs to access a service provided by one of these procedural operating systems, then the program must include a statement to make the appropriate operating system procedure call. This is the case, whether the software program is object-oriented, procedure-oriented, rule based, etc. Thus, conventional operating systems provide procedure-oriented environments in which to develop and execute software. Some of the advantages of OOT are lost when an object-oriented program is developed and executed in a procedure-oriented environment. This is true, since all accesses to the procedural operating system must be implemented using procedure calls defined by the operating system's native procedural interface. Consequently, some of the modularity, maintainability, and reusability advantages associated with object-oriented programs are lost since it is not possible to utilize classes, objects, and other OOT features to their fullest extent possible.
One solution to this problem is to develop object-oriented operating systems having native object-oriented interfaces. While this ultimately may be the best solution, it currently is not a practical solution since the resources required to modify all of the major, procedural operating systems would be enormous. Also, such a modification of these procedural operating systems would render useless thousands of procedure-oriented software programs. Therefore, what is needed is a mechanism for enabling an object-oriented application to interact in an object-oriented manner with a procedural operating system having a native procedural interface.